SphereGenerator (Take 2)

Sorry, the old pull request got trashed when I fixed some merge issues.

This should be in a good, pullable state. Sphere shows up nicely and normal generation seems to work fine. Note that I had to change all of the Indicies from `In16Array`s to `Uint16Array`s in order to get that to work, but I can't imagine any reason why you wouldn't want that anyway. Also, `Mesh` needed to have the type of `_vertexBuffers` changed to a `List<>`, which was odd. I'm not sure how you ever got the `BoxGenerator` to work without that...

Let me know if anything needs changing before you pull!